As a process for the preparation of a printing plate by use of electrophotography, there has so far been used a technique which comprises the steps of forming a toner image on a photoconductive layer of a photosensitive printing plate having a photoconductive layer on a support and, then, removing a non-toner portion of the photoconductive layer by dissolution development.
The foregoing technique, however, has a disadvantage that a dissolution solution permeates a photoconductive layer beneath an image portion where a toner image is formed during the dissolution development and, thereby, causes so-called side etching to remove this image portion; as a result, image reproducibility is impaired, or small dots and fine lines to be maintained are dissolved or removed.
In order to eliminate such a disadvantage, there is proposed a technique to give an electrophotographic photosensitive layer an exposure solubility difference by introducing a photo-solubilizable photosensitive compound into the electrophotographic photosensitive layer, and carrying out overall exposure after formation of a toner image (British Patent No. 996,315, Japanese Pat. O.P.I. Pub. Nos. 90648/1982, 150953/1983, 194467/1985, etc.). Further, formation of a toner image by use of a liquid developer is also known as a means to improve resolution (Japanese Pat. O.P.I. Pub. Nos. 90648/1982, 170862/1984, etc.).
It is known that electric charge per unit weight of toner in a liquid developer is about ten times that of a dry toner and thus it is difficult to give a high density in developing. When such a liquid developer is used in wet copying whose primary object is to obtain a hard copy on copying paper, a reflection density of about 1.2, and a transmission density of 0.5 or less are sufficient. When ordinarily used in printing plates, the toner of the liquid developer is not required to have a high transmission density, either, because visibility is not necessarily required as long as the toner functions as a resist in hydrophilicity providing treatment or dissolution removal of a non-image portion.
However, in the photomechanical process that uses a photo-solubilizable photosensitive compound and a photoconductor in combination to give an exposure solubility difference between image portions and non-image portions of a photoconductive layer by carrying out post exposure using a toner image as a mask, there is a problem that the transmission density of a toner layer formed by liquid development is not necessarily satisfactory as compared with that obtained by use of a dry toner.
When a photo-solubilizable photosensitive compound and a photoconductor are present in the same layer, a high toner density is especially required because a long post exposure time is necessary.
A low transmission density of a toner layer cannot give an adequate exposure solubility difference between image portions and non image portions and, thereby, lowers the dissolution latitude and reproducibility of small dots and fine lines.
Further, addition of a photo-solubilizable photosensitive compound in a large amount is necessary to impart a clear exposure solubility difference between image portions and non-image portions; therefore, a toner layer is required to have a transmission density high enough to allow an adequate post exposure in order to decompose thoroughly the photo-solubilizable photosensitive compound added in a large amount.
However, when the toner layer is made thicker to raise the transmission density, there arises another problem that the image reproducibility is lowered.